In airborne weather radar systems which emit a series of RF pulses the return signal (back scatter signal), as a function of time, is indicative of precipitation rate at increasing distances from the aircraft. The signal strength of the return signal for a given rate of precipitation is an inverse function of the distance from the aircraft. Said another way, the sensitivity decreases as a direct function of distance from the aircraft. Conventional radar systems employ circuitry which compensate for the reduction of signal strength such that a video signal produced by the compensating circuitry will be of constant value for a given rate of precipitation over a range of distances from the aircraft. The compensating network provides what is known as sensitivity time control (STC). The compensation for the distance is effective to a distance at which the assumed three nautical mile diameter target (e.g., area of precipitation) just fills the RF signal beam of the radar, known as beam filling.
For a given antenna, the distance is a given quantity. (The three nautical mile value is the assumed size of a target conventionally used by radar design engineers in their various design calculations). Beyond that distance STC is not functional and the return signal for a given rate of precipitation decreases as a function of distance from the aircraft, i.e., system sensitivity decreases.
In conventional radar systems, employing direct view storage tubes for display, the loss of sensitivity at distances beyond that at which STC is effective is not of great importance.
Modern radar systems, however, employ digital techniques and in particular typically include an analog-to-digital (A/D) converter for translating video signals produced by the compensating circuit into a four level code, the value of which, as a function of time, depends on the value of the video signal. For example, level three may represent heavy precipitation, level zero may represent no precipitation, while levels one and two represent intermediate conditions. Therefore, the video signal representative of conditions beyond that distance associated with STC (hereinafter, STC distance) will no longer give calibrated results. For example, a video signal representative of heavy precipitation which should be digitized as a level three signal, will be translated to a level two or level one giving the aircraft pilot a false indication of weather conditions beyond the STC distance. Such a radar is described in U.S. Pat. No. 4,086,579, issued Apr. 25, 1978, to F. C. Easter and assigned to the common assignee.
One solution would be to not display anything beyond the STC distance. But pilots want to "see" weather 100 to 300 nautical miles away, far beyond the STC distance which is typically on the order of thirty nautical miles. Another solution would be to increase the distance over which the compensating circuit is effective. This is, however, not always possible due to practical engineering problems.
The STC distance is limited by the antenna and receiver or system gain. Thus, the only way to extend this distance is to increase the gain. Not only is the gain limited on a technical basis, but also from a practical or cost effective basis.